1. Field of the Invention
This invention relates to a thermal transfer printer for color printing and, in particular, to a thermal transfer printer for color printing on recording paper by using an ink sheet having ink including three primary colors or four colors, one of which is black, in addition to the three primary colors.
2. Description of the Prior Art
A thermal transfer printer used as an output printer in computer systems, word-processors, and the like is capable of color printing relatively easily by applying ink including several colors several times to the same page of the same printing medium. Thus, this printer is suitable for outputting data displayed as the so-called computer graphics or as color images.
A thermal transfer printer capable of color printing as described above has been disclosed in, for example, the Japanese Patent Laid-open No. 58-140266 (1983).
In the thermal transfer printer of the kind referred to above, generally, an ink sheet having ink having four colors including three primary colors (yellow, magenta, and cyan) and black sequentially disposed thereon and a recording paper are put one upon another and are carried to a contact part between a thermal transfer head and a platen so that all colors of ink are thermally transferred to the recording paper in succession. After every time printing in one color is completed, the recording paper is carried backward and then printing in the succeeding color of the same page as that previously printed is performed, and the repeat of such a process results in color printing.
However, in the above-described conventional thermal transfer printer, a roll paper is usually used as the recording paper. But, since a reel for holding such a roll-shaped printing paper is freely rotatable within the printer, when the recording paper is carried in the backward direction, the roll of the recording paper is not rotated, whereby the recording paper sags. Strictly speaking, the recording paper begins to sag from the time when the recording paper begins to be carried in the backward direction and the sag of the recording paper is removed when the carrying of the recording paper in the forward direction for the thermal transfer printing by the ink of the next color is completed. In addition, the sag of the recording paper is at a maximum within a period of time from when the carrying of the recording paper in the backward direction is completed until the carrying of the recording paper in the forward direction is started, i.e., a period for awaiting the thermal transfer printing by the ink of the next color.
Accordingly, in the conventional thermal transfer printer, a space for accommodating the sagged portion of the recording paper produced during the carrying of the recording paper in the backward direction is required, whereby the printer as a whole is relatively large. Also, there is the possibility that the sagged recording paper can become caught on members within the printer to disturb the normal carrying of the recording paper. In addition, since the sagged recording paper is not controlled at all in carrying directions thereof (the forward direction or the backward direction), there is a great possibility that the recording paper will move obliquely when it is carried in the forward direction to carry out the thermal transfer printing.
Besides, there is the possibility that upon applying a great force to the recording paper when it is carried in the forward direction, the roll of the recording paper with the recording paper wound therearound will rotate excessively more than necessary due to the inertia thereof so that excessive recording paper is pulled at, whereby the recording paper becomes loose.
This applies also to the carrying of the recording paper in the backward direction. That is to say, also in the carrying of the recording paper in the backward direction, there is the possibility that upon applying a great force to the recording paper, the recording paper will be carried excessively more than necessary in the backward direction due the inertia thereof.
Besides, in the thermal transfer printer of this type, although it goes without saying that the thermal transfer head is pressed against the platen when the recording paper is carried in the forward direction to carry out the thermal transfer printing, the thermal transfer head is released from the platen when the recording paper is carried in the backward direction after the completion of the thermal transfer printing by the ink of one color. And, the thermal transfer head is again pressed against the platen when the carrying of the recording paper in the forward direction is started to carry out the thermal transfer printing by the ink of the next color. However, in the case where the recording paper sags between the thermal transfer head and the platen, whereby the recording paper does not contact the platen, the position of the recording paper relative to the platen is subtly changed every time the thermal transfer head is pressed against the platen. Accordingly, even though the amount of carrying the recording paper is carried in both the forward direction and the backward direction is strictly controlled, the position of the recording paper relative to the platen (the thermal transfer head) is changed every time the thermal transfer head is pressed against the platen at a point of time when the thermal transfer printing by the ink of each color is started, whereby color recording of high quality has been difficult to achieve.